A conventional envelope amplifier includes a linear amplifier, a switch amplifier, and a linear current detector. The linear amplifier is an independent voltage source which amplifies an envelope signal through a feedback network. The switch amplifier is a dependent current source which supplies the most amount of current required for an output current. The linear current detector detects the magnitude and direction of a linear current flowing from the linear amplifier to an output. The linear current detector changes the state (ON or OFF) of the switch amplifier to adjust a switch current.
The output current of the envelope amplifier includes a current flowing through the linear amplifier and a current flowing through the switch amplifier. The ratio of the linear current of the linear amplifier to the switch current of the switch amplifier serves as an important factor which determines the entire efficiency of the envelope amplifier.
When the switch current is smaller than the output current, a difference therebetween is added to the switch current by the linear current and then flows as the output current (source current). On the other hand, when the switch current is larger than the output current, a difference therebetween is subtracted from the switch current by the linear current and appears as the output current (sink current). The output current of the envelope amplifier corresponds to the sum of the switch current and the linear current. Thus, according to the efficiencies of the linear amplifier and the switch amplifier and the ratio of the switch current to the linear current, the entire efficiency of the envelope amplifier may be determined.
The conventional envelope amplifier uses a battery voltage as a supply voltage for the linear amplifier and the switch amplifier. However, since the battery voltage is changed according to the state of charge of the battery, the operations of the linear amplifier and the switch amplifier may be changed. In order to solve such a problem, the supply voltage of the linear amplifier may not be directly connected to the battery voltage, but a DC-DC converter may be used to provide a clamped supply voltage. The supply voltage of the switch amplifier may also be clamped through the DC-DC converter. However, the switch amplifier serves to supply a large amount of current to the entire output current. Thus, when the DC-DC converter is used, the entire efficiency may be degraded due to a power loss of the DC-DC converter.
In the envelope amplifier using the DC-DC converter, the DD-DC converter is used only in the linear amplifier. Thus, the supply voltages of the linear amplifier and the switch amplifier are asymmetrical with each other. Therefore, when the envelope amplifier is designed to be operated with the optimal efficiency at the maximum output power of the envelope amplifier, the envelope amplifier may not exhibit the optimal efficiency in case where the output power is lowered. This is because, since a customized switch current is designed to flow in case where the envelope amplifier has the maximum output power, the linear current and the switch current are set to the ratio at which the maximum efficiency can be exhibited. When a low input is applied, the amount of switch current is changed through the linear current amplifier.
However, due to the asymmetrical supply voltages of the linear amplifier and the switch amplifier, the amount of switch current is not automatically changed to the amount of switch current which maximizes the efficiency of the envelope amplifier, for low input power. Thus, as the output power of the envelope amplifier is changed from the maximum output power to low output power, the customized switch current does not flow. Furthermore, the efficiency may not be maximized in the entire output power region of the envelope amplifier.
The conventional envelope amplifier is designed to pass the switch current such that the envelope amplifier has the maximum efficiency at the maximum output power, without considering low output power. Thus, in the low output power region, the ratio of the linear current flowing through the linear amplifier and the switch current flowing through the switch amplifier may not correspond to the ratio at which the efficiency of the envelope amplifier is maximized.